What Gauge Means

A unit of measure of the internal diameter of a shotgun barrel, determined by the number of spherical lead bullets of a diameter equal to that of the bore that are required to make one pound... According to Webster's New Universal Unabridged Dictionary

That is because the 410 is actually the .410 caliber smooth bore. There are stranger things in this world of funny-named stuff. As an example, I have 3 36cal weapons, each of which takes a different size ball.